1. Eurasian J. Anal. Chem. 4(2): 204-214, 2009
A Spectrophotometric Method for the Determination of Nitrite and Nitrate
Badiadka Narayana1 and Kenchaiah Sunil
Department of P.G. Studies and Research in Chemistry, Mangalore University, Mangalagangotri-574
199, India
Received: 09 June 2009; Accepted: 26June 2009
Abstract
A simple, rapid spectrophotometric method for the determination of nitrite and nitrate in water, soil
and pharmaceutical preparation samples has been developed. Determination of nitrite is based on the
reactions involving sulfanilic acid with methyl anthranilate as the coupling agents and determination
of nitrate is based on their reduction to nitrite in the presence of Zn/NaCl. The produced nitrite is
subsequently diazotized with sulfanilic acid then coupled with methyl anthranilate to form an azo dye
which is measured at 493 nm. The method is optimized for acidity, amount of reagents required and
tolerance amount of other ions. The range of linearity for sulfanilic acid-methyl anthranilate couple
was found to be 0.2-8.0 µg/mL of nitrite with molar absorptivity be 1.03x104 Lmol-1cm-1 and sandell’s
sensitivity 4.5x10-3 µg cm-2 . The detection limit and quantitation limit of the nitrite determination are
found to be 0.93 µgmL−1 and 2.82 µgmL−1 respectively. This method has been successfully applied to
the determination of trace amounts of nitrite and nitrate in water, soil and pharmaceutical preparations.
Keywords:
Spectrophotometry; Diazotization; Nitrite; Nitrate; Methyl anthranilate; Isosorbide
1. Introduction
Nitrite is a versatile chemical agent which has found numerous applications ranging
from dye manufacture to food preservation. It produces carcinogenic nitrosamines in the
human body through its reaction with amines or amides [1]. Nitrite is one of the pollutants
found in the atmosphere and natural water [2] and is an important intermediate in biological
nitrogen cycle. Traces of nitrite and nitrate in drinking water may lead to mathemeglobmenia
in infants and with long term exposure is a possible cancer risk. Various instrumental methods
such as polarography [3], voltammetry [4], fluorimetry [5], biamperometry [6] and flow
injection spectrophotometry [7] have been used for nitrite determination. Nitrite is determined
spectrophotometrically based on diazo coupling reaction [8, 9] extraction of the azo dye into
suitable organic solvent provides a much lower detection limit and improved sensitivity [10].
Nitrate is a well-known contaminant of ground and stream water. It is an important
environmental and human health analyte, and thus its detection and quantification are
considered to be essential. An excellent review on the detection and determination has been
reported by Moorcroft et al [11]. Most of the recent work concerning nitrate determination
has embraced the classical reagents. Several reported spectrophotometric methods involve the
use of common reactions, such as a reduction reaction followed by diazotization, [12, 13]
nitration reactions, [14] or others [15,16]. Other methods involve the use of ion
chromatography [17] and specific ion electrodes [18]. The well-known spectrophotometric
1
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ISSN: 1306-3057 Moment Publication ©2009

2. Narayana and Sunil
methods for the determination of nitrate are based on the nitration of phenolic compounds
[19], chromophoric acids [20], 2,4-xylenol [21], 2,6-xylenol [22], 3,4-xylenol [23],
phenoldisulfonic acid [24], brucine[25] and phenol [26] 4-aminoazobenzene [27]. Some
sensitive spectrophotometric methods for determine nitrate utilize extractable ion associates
of the nitrate ion with basic dyes, like crystal violet [28] and nile blue [29].
In this work, a simple and rapid method has been proposed for the determination of
nitrite using methylanthranilate as a coupling agent. Sulfanilic acid was diazotized in acidic
medium and coupled with methylanthranilate to give a colored dye having absorption
maximum at 493 nm. Determination of nitrate is based on the reduction of nitrate to nitrite in
the presence of Zn/NaCl. The produced nitrite is subsequently diazotized with sulfanilic acid
and then coupled with methylanthranilate to form an azo dye and was measured at 493 nm.
The developed method has been successfully applied to the determination of nitrite and nitrate
in different samples. The comparisons of spectrophotometric methods for the determination of
nitrite with proposed method are summarized in Table 1.
Table 1. Comparison of spectrophotometric methods for the determination of nitrite with proposed
method.
Range λmax Molar absorptivity
Reagent Remarks Ref. No Remarks
μg mL-1 nm (L mol-1 cm-1)
Extractive and
Neutral red [30] 0-20 530 2.50 x 104 common ions
interfere.
Less stable and
Leucocrystal violet [31] 0.004-0.04 500 1.54 x 104 less detection
limit
Phenosafranine [32] 0-12 520 1.03 x 104 Less sensitive.
PNA+ Time consuming
[33] 0.05-0.80 500 1.45 x104
diphenylamine and less sensitive
pH dependent
PNA+8 quinollinol [34] 0.01-0.06 550 3.88 x 104 and time
consuming
pH dependent
MMA+ N,N and time
[35] 0.05-2.0 482 2.03 x 104
dimethy- and Aniline consuming low
detection limit
Less sensitive
SA+EAA [36] 0.2-3.0 356 1.22 x 104
and less stable.
Simple, rapid,
Sulfanilic acid + [present non extractive,
0.2-8.0 493 1.03x104
Methyl antranilate method] high sensitive and
stable.
PNA: p-nitroaniline; MMA; 4-(1-Methyl-1-mesitylcyclobutan-3-yl)-2-aminothiazole; SA: sulfanilamide; EAA:
ethyl acetoacetate
2. Experimental
2.1 Apparatus
A SHIMADZU (Model No: UV-2550) UV-Visible spectrophotometer with 1 cm
matching quartz cells were used for the absorbance measurements. A WTW pH 330 pH meter
was used.
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3. Eurasian J. Anal. Chem. 4(2): 204-214, 2009
2.2 Reagents and Solutions
All chemicals used were of analytical reagent grade, and doubly distilled water was
used in the preparation of all solutions in the experiments. Nitrite solution (1000 gmL-1) was
prepared by dissolving 0.1500 g sodium nitrite in water and diluting to 100 mL. Nitrate
solution (1000 gmL-1) was prepared by dissolving 0.7220 g potassium nitrate in water and
diluting to 100 mL. Working standard solutions were prepared by appropriate dilution.
Sulfanilic acid (0.5 g in 100 mL water) and methyl anthranilate (0.5 mL in 100 mL of
alcohol) were used. The following reagents were prepared by dissolving appropriate amounts
of reagents in water: 2 mol L-1 HCl & 2 mol L-1 NaOH.
2.3. Procedure
2.3.1. Nitrite determination
Aliquots of stock solution containing 0.2-8.0 µgmL-1 of nitrite were transferred in to
series of 10 mL calibrated flask. To each flask, 1 mL of 0.5% sulfanilic acid and 1 mL of 2
mol L-1 hydrochloric acid solution were added and the solution was shaken thoroughly for 5
min to allow the diazotization reaction to go to completion. Then, 1 mL of 0.5% methyl
anthranilate and 2 mL of 2 mol L-1 sodium hydroxide solution were added to form an azo dye
and the contents were diluted to 10 mL using water. After dilution to 10 mL with water,
absorbance of the red colored dye was measured at 493 nm against the corresponding reagent
blank and the calibration graph was constructed. The results are summarized in Table 2.
Table 2: Determination of nitrite in water and soil samples using methyl anthranilate as a reagent
Proposed method Reported method [30]
Nitrite
Sample added Nitrite Nitrite t-testb F-testc
Recovery Recovery
μgmL-1 found in found in μgmL-
-1 a % 1 a %
μgmL ± SD ± SD
2.0 1.99 ± 0.04 99.50 1.98 ± 0.03 99.00 0.56 1.80
Tap
4.0 4.01 ± 0.04 100.75 4.01 ± 0.05 100.75 0.57 1.71
water
6.0 5.98 ± 0.03 99.66 6.01 ± 0.03 100.16 1.49 1.12
2.0 1.99 ± 0.04 99.50 2.01 ± 0.04 100.50 0.56 1.28
River
4.0 3.99 ± 0.04 99.75 3.99 ± 0.04 99.75 0.56 1.23
water
6.0 5.99 ± 0.03 99.83 5.99 ± 0.05 99.83 0.74 1.97
2.0 1.98 ± 0.03 99.00 1.99 ± 0.04 99.50 1.49 1.80
Ground
4.0 4.01 ± 0.05 100.25 4.01 ± 0.04 100.25 0.44 1. 71
water
6.0 6.01 ± 0.03 100.16 5.98 ± 0.03 99.66 0.74 1.12
2.0 2.01 ± 0.04 100.50 1.99 ± 0.04 99.50 0.57 1.28
Soil
4.0 3.99 ± 0.04 99.75 3.99 ± 0.04 99.75 0.56 1.23
Sample
6.0 5.99 ± 0.05 99.83 5.99 ± 0.03 99.83 0.45 1.97
a
Mean ± Standard deviation (n = 5)
b
Tabulated t-value for 8 degrees of freedom at P(0.05) is 2.65
c
Tabulated F- value for (4,4) degrees of freedom at P (0.05) is 5.72
2.3.2. Nitrate determination
Pipetted out 10 mL of nitrate stock solution to a beaker, added 5 mL of Conc. HCl and 2
mL of Zn/NaCl granular mixture0, and was allowed to stand for 30 minutes. with
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4. Narayana and Sunil
occasionally stirring to form nitrite, then the solution was filtered to 100 mL standard flask
using Whatman No 41 filter paper and diluted up to the mark. Aliquots of stock solution
containing 0.26-10.7 µgmL-1 of reduced nitrate were transferred in to series of 10 mL
standard flask. Added 1 mL of 0.5% sulfanilic acid and 1 mL of 2 mol L-1 HCl solution,
shaken thoroughly for 5 minutes for the diazotization reaction to go to completion. Then, 1
mL of 0.5% methyl anthranilate and 2 mL of 2 mol L-1 sodium hydroxide solution were added
to form an azo dye and the contents were diluted to 10 mL with water. After dilution to 10 mL
with water, the absorbance of the red colored dye was measured at 493 nm against the
corresponding reagent blank. The results are summarized in Table 3.
Table 3: Determination of nitrate in water and soil samples using methyl anthranilate as a reagent
Proposed method Reported method [30]
Nitrite
Nitrite Nitrite
Sample added Recovery Recovery t-testb F-testc
found in found in
μgmL-1 -1 a % -1 a %
μgmL ± SD μgmL ± SD
4.0 3.90 ± 0.04 97.50 3.93 ± 0.04 98.25 0.66 1.71
Tap
6.0 5.91 ± 0.04 98.50 5.96 ± 0.04 99.33 0.89 1.23
water
8.0 7.92 ± 0.03 99.00 7.95 ± 0.05 99.37 1.49 1.02
4.0 3.92 ± 0.04 98.00 3.90 ± 0.04 97.50 0.46 1.28
River
6.0 5.93 ± 0.04 98.83 5.91 ± 0.04 98.50 1.06 1.81
water
8.0 7.95 ± 0.03 99.37 7.92 ± 0.03 99.00 0.74 1.26
4.0 3.96 ± 0.03 99.00 3.92 ± 0.04 98.00 1.49 1.80
Ground
6.0 5.92 ± 0.05 98.66 5.93 ± 0.04 98.83 0.56 1. 71
water
8.0 7.92 ± 0.03 99.00 7.95 ± 0.03 99.37 0.89 1.12
4.0 3.93 ± 0.04 98.25 3.96 ± 0.03 99.00 0.87 1.58
Soil
6.0 5.96 ± 0.04 99.33 5.92 ± 0.05 98.66 1.49 1.81
Sample
8.0 7.95 ± 0.05 99.37 7.92 ± 0.03 99.00 0.56 1.26
a
Mean ± Standard deviation (n = 5)
b
Tabulated t-value for 8 degrees of freedom at P(0.05) is 2.65
c
Tabulated F- value for (4,4) degrees of freedom at P (0.05) is 5.72
2.4. Nitrite and Nitrate Determination in Soil Sample
About 1.0 g of soil sample was taken in a 25 mL beaker and extracted with three mL
portions of 0.5% sodium carbonate solution. The extract was filtered through Whatman no. 41
filter paper. The filtrate were collected and diluted to 25 mL. Appropriate aliquots of 1-2 mL
of the solution was transferred in to a 10 mL calibrated flask and analyzed according to the
general procedure. They tested negative. To these samples known amounts of nitrite and
nitrate sample were added and analyzed for nitrite and nitrate following the proposed
procedure. The results are summarized in Table 2 and 3.
2.5 Nitrate Determination in Pharmaceutical Samples
Isosorbide dinitrate (0.05 g) sample was taken, dissolved in water and clear solution was
made up to 100 mL. Known amount of this solution was taken and analyzed for nitrate
content following the procedure described for the analysis of water sample. Isosorbide
mononitrate sample was also analyzed by the same procedure. The results are summarized in
Table 4.
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5. Eurasian J. Anal. Chem. 4(2): 204-214, 2009
Table 4: Determination of nitrate in pharmaceutical preparations using sulfanilic acid-Methyl
anthranilate as reagents
Proposed method Reported method [30]
Nitrate
Nitrate Recover Nitrate
Sample added Recovery t-testb F-testc
found in y found in
μgmL-1 %
μgmL-1± SDa % -1
μgmL ± SD a
IDd -- 1.93 ± 0.05 -- 1.94 ± 0.05 -- 0.44 1.04
(0.05 g) 5 6.92 ± 0.05 98.00 6.94 ± 0.06 99.50 0.89 1.44
IMe -- 1.21 ± 0.04 -- 1.19 ± 0.04 -- 0.55 1.06
(0.05 g) 5 6.20± 0.03 98.50 6.17 ± 0.04 96.50 0.74 1. 77
a
Mean±Standard deviation (n = 5). b Tabulated t-value for 4 degrees of freedom at P(0.05) is 2.78.c Tabulated
F44 degrees of freedom at P (0.05) is 5.72. d Sample taken 2 μg nitrate (certified). Nichoals Piramal Intra
Limited, Himachal pradesh. e Sample taken 1.25 μg nitrate (certified). Sun pharmacetical Industries, India.
3. Results and Discussion
3.1. Absorption Spectra
This method is based on the diazotization of sulfanilic acid in acid medium followed by
the coupling with methyl anthranilate in alkaline medium, which gives an azo dye with
absorption maximum at 493 nm. Diazotization and coupling reactions are found to be
temperature dependent. The absorption spectrum of the colored species of azo dye is
presented in Fig. 1 and the reaction system is presented in the following scheme.
- HCl + -
NO 2
HO 3S NH2 + HO 3S N2 Cl
O OCH3
NH2
+ -
O
HO3S N2 Cl + HO3S N
N NH OCH3
Fig.1. (a) Absorption spectra of azo dye; sulfanilic acid -methyl anthranilate couple and (b)
reagent blank vs distilled water
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6. Narayana and Sunil
3.2. Effect of Reagent Concentration
A volume of 1 mL of 0.5% sulfanilic acid solution was required for maximum
absorbance (Fig.2) and it is found that, addition of 1 mL of methyl anthranilate (0.5%) reagent
provides maximum absorbance (Fig.3) The use of larger excess of reagent produced no
further increase in the absorbance. Diazotization and coupling reactions are found to be
temperature dependent. Diazotization is carried out in cold condition (about 0-50C) and
coupling reaction was carried out at room temperature, above 350C there was a decrease in
intensity of the color (Fig.4). When the acid concentration is increased above 2 mol L-1 does
not affect the absorbance.
Fig.2: Absorption spectra; Variation of Sulfanilic acid
Fig.3. Absorption spectra; Variation of Methyl anthranilate
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7. Eurasian J. Anal. Chem. 4(2): 204-214, 2009
Fig.4. Absorption spectra; Variation of Temperature
3.3. Analytical Data
In this method adherence to Beer’s law is studied by measuring the absorbance values
of solutions varying nitrite concentration. A straight line graph is obtained by plotting
absorbance against concentration of nitrite. Beer’s law is obeyed in the concentration range
0.2–8.0 µgmL−1 of nitrite. Adherence to Beer’s law graph for the determination of nitrite
using methyl anthranilate is presented in Fig.5. The molar absorptivity and Sandell’s
sensitivity of the method is found to be 1.03x104 L mol-1 cm-1 and 4.5x10-3 µgcm−2. The
correlation coefficient, detection limit (DL=3.3σ/S) and quantitation limit (QL=10σ/S) (where
σ is the standard deviation of the regent blank (n=5) and ‘S’ is the slope of the calibration
curve) of the nitrite determination are found to be 0.992, 0.93 µgmL−1 and 2.82 µgmL−1
respectively.
Fig.5. Adherance to beer’s law for the determination of nitrite using methyl anthranilate as a
reagent
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8. Narayana and Sunil
3.4. Effect of Divers Ions
The effect of various non-target species on the determination of nitrite and nitrate were
investigated. The tolerance limits of interfering species were established at those
concentrations that do not cause more than ± 5% error in absorbance values of nitrite (2
µgmL−1) and nitrate (2.6 µgmL−1) with fixed concentration. The studies revealed that Ce(IV)
and Hg(II) showed severe interference. However, the tolerance levels of these ions are
increased by the addition of 3 mL of 2% EDTA. The results are given in table 5.
Table 5. Effect of diverse ions on the determination of nitrite (2 µgml−1) and nitrate (2.6 µgml−1) using
methyl antranilate as a reagent
Diverse ions Tolerance limit (µgmL−1)
Al3+ 300
Ba2+ 200
Ca2+ 500
Cd2+ 200
Ce4+* 25
CHCOO− >2000
citrate 800
Cu2+* 25
Fe3+* 25
Hg2+* 25
K+ >2000
Mg2+ 500
Mn2+ 500
Mo6+* 25
Na+ >2000
oxalate 800
Sn2+* 25
Pb2+* 25
tartarate 800
W6+* 25
* Masked by EDTA masking agent
4. Applications
The proposed method is applied to the quantitative determinations of nitrite and nitrate
in soil and water samples. Statistical analyses of the results by t- and F-tests show that, there
is no significant difference in accuracy and precision of the proposed and reported method
[30]. The precision of the proposed method is evaluated by replicate analysis of samples
containing nitrite and nitrate at five different concentrations (Table 2 and 3).The reagents
provide a simple and sensitive method for the spectrophotometric determination of nitrite and
nitrate. The proposed method has been successfully applied to the determination of trace
amounts of nitrite and nitrate in soil and water samples. This method has (Table 2 and 3) been
successfully applied to the determination of trace amounts of nitrate in pharmaceutical
preparations.
5. Conclusions
The reagents provide a simple and sensitive method for the spectrophotometric
determination of nitrite and nitrate. The reagents have the advantage of high sensitivity and
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9. Eurasian J. Anal. Chem. 4(2): 204-214, 2009
low absorbance of reagent blank. The developed method does not involve any stringent
reaction conditions and offers the advantages of color stability about more than 4 hours. The
proposed method has been successfully applied to the determination of trace amounts of
nitrite and nitrate in soil and water samples. The proposed method has been successfully
applied to the determination of trace amounts of nitrate in pharmaceutical preparations.
Acknowledgement
The authors are thanking full to DST, Government of India for the financial assistance
through FIST programmer. Authors also thank the Department of Studies in Chemistry,
Mangalore University for research facilities.
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